1. Field of the Invention
The embodiments of the invention generally relate to signal processing and, more particularly, to techniques for signal acquisition and tracking.
2. Background
Global positioning systems (GPS) have found wide application for precision spacecraft navigation and formation flying applications in Low Earth Orbits (LEO), but recent advances in GPS receiver designs and signal processing capabilities now make it feasible to consider using GPS to provide autonomous, onboard navigation capabilities for geostationary (GEO) or other high altitude space missions.
Conventional (GPS) receivers are generally not readily capable of operating effectively at altitudes above LEO. This is primarily due, in part, to the fact that the GPS signals available there are approximately 10-100 times weaker much more sparsely present than on Earth or at LEO. However, GPS receivers typically provide a very accurate and economical means of navigation and have become very popular for LEO missions and could be just as popular for High Earth Orbit (HEO-above LEO) missions if the technology were available.
Generally, GPS signal processing can be divided into two major tasks: signal acquisition (or detection) and then signal tracking (and demodulation). Signal acquisition is the process of finding a GPS signal within a two-dimensional unknown parameter space. Signal tracking is the process of continuously updating the estimates of these two and other signal parameters. Signal acquisition is generally a more difficult process than signal tracking, and it is the inadequacy of conventional GPS acquisition techniques that generally prevents the use of weak signals in GPS receivers.
The generally inadequate (for weak signals) conventional approach is to employ a serial search of the two-dimensional parameter space during acquisition. Typically, the same hardware that is used in signal tracking is reconfigured to affect the search. During a cold-start, which is a lack of any prior (a priori) information about visible GPS signals, acquisition by serial search can take upwards of 20 minutes for a very strong signal. To acquire weak signals, more data must be examined. Approximately 10 times the data record length is required for a 10 times weaker signal. Using serial search methods acquisition times grow quadratically, which results in a 20-minute search increasing to approximately 33 hours, which practically means that the signal cannot be acquired at all. Accordingly, there remains a need for a new, fully space-qualified weak signal GPS receiver targeted for high altitude applications.